Precise mutual positioning of two (or more) articles and/or their alignment are needed in a number of situations. For instance, two capillaries have to be aligned to secure smooth through-flow of liquid; two optical fibers are aligned to secure transmission of light in communication cables; two tubes are aligned for precise coaxial welding; two electrical connectors are aligned to secure conduction between corresponding contacts; and so on.
In other cases, the mutual distance of two articles or their parts is important as well, such as in the case of optical elements (lenses, prisms, mirrors etc.) assembled into an optical apparatus.
Such alignment or positioning is more difficult if the articles are very small and/or if the requirement on the precision of the alignment is very high. The positioning can be temporary in some cases and permanent in others.
The precise mutual positioning can be achieved by a number of methods of mechanical or optical nature which require very precise, expensive instrumentation and considerable skill.
An example of such a precise positioning and aligning method is fusion of two optical fibers by heat. Prior to the fusion, the two glass fiber ends have to be manipulated into a precisely coaxial position a very small distance apart. Because the fibers are typically around 100 microns in diameter and have to be positioned with micron or sub-micron precision in all three coordinates, the whole work has to be done under stereo-microscope by very precise X-Y-Z manipulators. Such methods are hardly suitable for inexpensive mass production, field repair etc. One group of methods used for precise positioning of optical fibers or electrical contacts utilizes so called "Memory Materials". Memory materials are either plastics or metals which have several common features:
a) Memory materials have two distinct types of behavior in two different temperature regions separated by a certain "transition temperature" Ts. (Namely, they are substantially more deformable above Ts than below Ts.)
b) Memory materials have certain "inherent shapes" into which they tend to return above Ts due to internal stresses (in substantial absence of external stresses). This shape is "inherent" (or exactly defined) with certain resolution or precision.
c) Memory materials can be deformed into a shape which differs from the said Inherent shape. The Memory Materials in Deformed Shape can hold internal stresses for very long time periods at temperatures below Ts.
Typically, the Memory Materials for the positioning application are manufactured in their Inherent Shape corresponding to the desired mutual positions of the two articles. Then the memory material is heated above Ts and deformed into a shape convenient for insertion of the articles, their assembly, etc.; and fixed in such "deformed shape" by decreasing temperature below the material's Ts.
The mutual positioning is then done in reverse order, using the Memory Material to return to its Inherent Shape (which corresponds to the desired positions of the parts) after being heated above Ts for a sufficient length of time. The methods using Memory Materials were described mainly for electrical connectors and splicing optical fibers.
Typically MEMORY MATERIAL SPLICERS have a continuous cavity which has an inherent diameter smaller than the diameter of the spliced fiber. Prior the splicing, the cavity size is increased by mechanical pressure above Ts and then fixed in its new (i.e. deformed) shape by decreasing temperature below Ts. The enlarged cavity can then readily accept the fiber ends, and the memory material is then heated above Ts so that the cavity collapses around the fiber forcing its ends into alignment and immobilizing the fibers at the same time.
Various modifications of this process and various "heat shrinkable" materials can be employed.
One group of such materials are Memory Metals described e.g. in U.S. Pat. Nos. 4,261,644; 4,352,542 and 4,597,632 or British Patent No. 1,555,475.
Another known group of Memory Materials are covalently crosslinked crystalline polymers known also as "heat shrinkable plastics" etc. and described e.g. in the U.S. Pat. Nos. 3,086,242; 3,359,193; 3,370,112; 3,597,372 and 3,616,363. Their use for connectors and splicers is described for instance in U.S. Pat. Nos. 4,178,067; 4,489,217 and 4,725,117.
There are several problems with the hitherto used memory materials. Memory metals have high density, they are not transparent and their deformation (particularly increase of the internal size of the cavity) is difficult to achieve. They are also made and processed at high temperatures which only a few mandrel or mold materials can withstand. They can be deformed only to a relatively small degree.
Hitherto known crystalline memory polymers have a different set of problems. First of all, crystalline memory polymers described so far are relatively soft at ambient condition because their amorphous phase has Tg lower than ambient temperature. Therefore, they cannot usually hold the articles positioned by themselves and require rigid support structures made of other materials.
Secondly, their Ts correspond to melting temperature of the crystalline phase and this phase transition prevents precise definition of the Inherent Shape. The softening and solidification of this type of memory polymer is caused by melting and recrystallization of the crystalline phase. The crystalline phase morphology defines the solidified shape and the crystalline morphology is, in turn, affected by external stresses. Therefore, there is no single unambiguous "inherent shape" below the melting temperature.
In addition, the crystallization is accompanied by substantial volume changes which may interfere with the alignment.
The known turbidity of crystalline polymers is also a disadvantage in some applications. Amorphous Memory Polymers (AMP) were not used so far for connecting or positioning of articles. Their memory properties and optical clarity were utilized for Intra-Ocular Lenses insertable through a small incision as described in U.S. Pat. No. 4,731,079. However, the Amorphous Memory Polymers for the intraocular lens have too low a Ts to be very useful in positioning or aligning articles in most cases.
AMP was also used for a simple device useful for splicing of optical fibers as described in the co-pending U.S. patent application Ser. No. 07/288,629 (U.S. Pat. No. 4,921,323).